Finally, we will identify the muscles of the posterior compartment of the forearm. If we consider the nerve supply of the muscles of the posterior compartment of the forearm (radial), then all of the muscles supplied by that nerve should be located in that compartment. However, one of the muscles has migrated anteriorly and is best seen from that vantage point. The muscle is the brachioradialis.

brachioradialis muscle

The remainder of the muscles of the posterior compartment can be examined in three groups: 1) superficial2) intermediate3) deep.

For the most part, the superficial and intermediate groups arise from the lateral epicondyle of the humerus.

Muscles of the Posterior Compartment of the Forearm

Superficial group

extensor carpi radialis longus

extensor carpi radialis brevis

extensor carpi ulnaris

Intermediate group

extensor digitorum

extensor digiti minimi

Deep group

anconeus

supinator

abductor pollicis longus

extensor pollicis brevis

extensor pollicis longus

extensor indicis

The Extensor Expansion

Once the muscles of the posterior compartment are identified and studied, take a look at the insertions of the tendons of the extensor digitorum. They are special and deserve closer attention.

The extensor tendons insert into the phalanges by way of a special connective attachment called the extensor expansion.

Dorsal View

Lateral View

When the extensor digitorum muscle contracts, it pulls on the extensor expansion and this, in turn extends the interphalangeal joints. You will also notice that the lumbrical muscle inserts into the hood part of the expansion and through this attachment can also extend the interphalangeal joints. This muscle can at the same time flex the metacarpophalangeal joint. Because of these two actions, the lumbricals are called the bye bye muscles. That is the action they perform.